Never. We always do all 4 main parks every trip. We always go for at least 7-8 days and sometimes longer.
There is something in every park that someone in my family of 6 likes to do, so we hit them all up. We go every 12-18 mos, so that is a long time in between if we didnt do all parks.
I can see if you had a shorter trip that it may be better to skip a park, but we do them all